1. Field of the Invention
The present invention relates to signal processing circuits as may be used in a radio receiver and more particularly, to such circuits as may be used in a mobile radio telephone capable to prevent collision and omission of received signals when a waveform equalizer is to be selectively used for radio signals from a base station.
2. Description of the Related Art
A time division multiple access system is used in a digital mobile radio telephone system which consists of a plurality of stationary base stations and a plurality of independent mobile stations. Some mobile stations have waveform equalizers in their receiving circuits so as to remove multi-pass fading or compensate for data omission due to noise or the like for radio signals from the base stations. However, since the use of this waveform equalizer results in increased consumption of electric power, refrainment from its use as far as possible is preferred for the mobile stations which derive power supply from batteries or the like. For this reason, a mobile station which switches between the use and non-use of the waveform equalizer is described in, for example, U.S. Pat. No. 5,230,077.
In this mobile station, the omission or collision of signals occurs when switching is done between use and non-use of the waveform equalizer. The reason is that a waveform equalized signal to a decoder is delayed by the time taken to effect the equalization processing, compared with a waveform non-equalized signal. For example, when a signal supplied to the decoder is switched from the equalized signal to the non-equalized signal, the equalized signal overlaps the non-equalized signal because the equalized signal is delayed and the non-equalized signal is not delayed. Therefore, the decoder runs into collision and thereby invites the loss of data. On the other handy when the signal supplied to the decoder is switched from the non-equalized signal to the equalized signal, the input of the decoder is interrupted since the start of use of the waveform equalizer invites an initial delay of received signal.